The invention relates to synthetic cages for rolling bearings.
These cages are in the form of an annular element with a certain number of cells intended to accommodate the rolling bodies, for example balls, and space them correctly and uniformly in the circumferential direction with respect to one another and thus avoid any imbalance as the bearing rotates.
The cells are generally bordered, at least on two opposite sides in the circumferential direction, with retaining tabs intended to hold the rolling bodies in the cells and, during operation, hold the cage in place on the row of rolling bodies.
The cage is therefore mounted on the rolling bodies (or the rolling bodies are mounted in the cells of the cage) by clipping the rolling bodies and cells together using the flexibility of the retaining tabs.
Thus, in the case of a rigid ballbearing, the retaining tabs extend the cells in such a way as to define an opening, the diameter of which is slightly smaller than that of the ball.
The row of balls is first of all mounted between the outer ring and the inner ring of the bearing. The balls are then spread out circumferentially using an appropriate tool.
The cage is positioned over the balls in such a way that each cell faces each ball.
The cage is pushed axially towards the row of balls, which causes the retaining tabs to open up, these then closing up again over the balls by elasticity as soon as these balls have adopted the correct position in the spherical cells.
The cage is then centred radially with respect to the rings by the spherical cells and is held axially on the said row of balls by the retaining tabs belonging to the cells.
Such cages are known per se and have been used for many years.
These cages, by comparison with metal cages, have the advantage of a low production cost and of having a low mass and therefore low inertia.
The good slip properties of plastic on steel also encourages running with little friction and therefore little wear and so generates little heat and consumes little energy.
Finally, such types of cage also play a part in making the bearing run more quietly.
These cages are mass-produced by injection moulding a material such as polyamide (PA), possibly filled with glass fibres in order to improve its mechanical properties.
The main parameters which limit the possibility of using synthetic cages in bearings are the running temperature and the ageing that is due to chemical attack from the lubricant.
Thus, in the case of a cage made of nylon-6,6, reinforced with glass fibres, the operating temperature is limited to 120xc2x0 C. in continuous use.
This limit can be pushed back by using cages made of better materials such as polyphenylene sulphide (PPS) resins with a crosslinked structure, polyether sulphone (PES) resins, polyetherimide (PEI) resins or polyetheretherketone (PEEK) resins, but these materials exhibit two major drawbacks:
high cost price
lack of flexibility making the cage difficult to release from the mould and also making it difficult for the rolling bodies to be mounted in the cells (risk of breaking the tabs).
It might also be possible to envisage a polymer from the polyphthalamides (PPA) group, but PPA is a material with low ductility which cannot therefore be used to obtain a cage whose retaining tabs are flexible enough to allow easy release of the said cage from the mould and to allow the rolling bodies to be mounted in the cells without the risk of breaking the latter.
The invention sets out to solve all of these problems.
The object of the invention is to produce a cage for high temperatures, which is chemically resistant to lubricants, has flexible retaining tabs and is of low cost.
The manufacturing method according to the invention is intended for cages of the type intended to hold rolling bodies in place in a rolling bearing comprising at least one row of rolling bodies arranged between two raceways. The said cage comprises an annular part with cells intended to accommodate the rolling bodies, the said rolling bodies being clipped into the cells and held therein by means of elastically deformable members. The cage is obtained by moulding a blend of a polymer from the polyphthalamides group and of an elastomer.
As a preference, the said polymer from the polyphthalamides group is obtained by the polycondensation of hexamethylenediamine, terephthalic acid, isophthalic acid and adipic acid.
In one embodiment of the invention, the said blend is filled with glass fibres, the glass fibres being incorporated into the blend prior to moulding.
Another subject of the invention is a cage for rolling bodies of a rolling bearing, comprising a polymer from the polyphthalamides group and an elastomer which are blended together, and a rolling bearing equipped with such a cage.
As a preference, the melting point of the material of which it is made is 300xc2x0 C. or higher.
In one embodiment of the invention, the glass transition temperature of the material of which it is made is 130xc2x0 C. or higher.
In one embodiment of the invention, the uppermost temperature for continuous use is 140xc2x0 C. or higher when used in contact with a lubricant.
In one embodiment of the invention, the ductility of the material of which it is made is 5% or higher.
In one embodiment of the invention, the bending strength of the material of which it is made is 260 N/mm2 or higher.
In one embodiment of the invention, the impact strength of the material of which it is made is 50 kJ/m2 or higher.
The rolling bodies are balls or rollers.
The material obtained according to the invention has a semi-crystalline structure and good properties in respect of:
highest temperature compatible with the chemical resistance to the various lubricants used in the bearing,
ductility,
flexibility,
fatigue strength,
impact strength,
dimensional stability.